Oil in power transformers is regularly filtered every twelve to eighteen months to remove impurities which build up during operation. The impurities can include carbon as well as other substances that cause an insulating property breakdown of the oil. The filter medium generally used is classified as a montmorillonite clay. This type of clay has an expanding structure with high adsorptive powers. The material is crystalline and non-plastic similar to fuller's earth which is a natural fine grained material. Throughout the text the oil soaked filter medium is referred to as "filter cake".
After the transformer oil has been filtered, the oil remaining in the filter cake can be higher than 30% of the total weight. In the past this filter cake has been disposed of in landfills with consequent leaching out of the oil to the subsoil, water table and surface streams. Today many landfills are not accepting this highly toxic material so that utility companies store the spent filter cake awaiting a treatment solution. It is known for example that in Ontario, Canada, some 2,000 tons of filter cake are generated annually. It is believed that the amount of stored filter cake in both the United States and Canada is considerable.
Remediation methods for toxic materials are disclosed in U.S. Pat. No. 5,209,851 issued May 11, 1993 to Hume and Downie. The process disclosed herein relates to treating toxic waste materials with protein nutrients to remediate the contaminated material and efficiently reduce the toxic conditions to innocuous levels. A method of treating waste water utilizing an aerating process is disclosed in Canadian Patent 1,182,227 issued Feb. 5, 1985 to Hume. In this process effluent is treated such that substantially no sludge is formed. Toxic effluent is first skimmed to remove floating oil and other solids. The effluent is then vigorously mixed and aerated so the oily material present in the effluent is maintained in suspension such that minimum flocculation occurs. The mixing and aeration continues for a sufficient period of time to permit remediation of the carbon impregnated oily waste water.